This invention relates to jet finishing of a continuous web passing through a coating pot to control the thickness of the coating and to be able to leave coatings of different thicknesses. More particularly, the invention relates to an assembly having rotatably mounted jet knives having different orifice openings for providing different coating thicknesses.
It is well known to use jet finishing knives for controlling the thickness of liquid coatings in hot dip metal coating processes for metals such as zinc and aluminum and in the coated paper and film industries. The liquid coating remaining on the metal strip, film or paper web, all three herein being referred to as webs, must be uniformly controlled across the width and along the length of the web to obtain a satisfactory product. The major problem when scheduling production on a coating line, particularly in the steel industry, is to schedule extended production runs for material to receive the same class of coating weights, i.e. same thickness. This means much material must remain in inventory for extended periods of time because the coating ordered does not match that of the current production schedule. This not only increases costs for the manufacturer, but also increases costs for the customer because both must maintain larger inventories. Furthermore, an extended line stoppage is required to change nozzle size when scheduling a different class of coating weight.
A different but related problem occurs when producing two side differential coatings. A differentially coated galvanized steel strip typically has a thin alloyed zinc coating on one side of the strip and a thick unalloyed zinc coating on the other side of the strip. Switching a production schedule for producing two side coatings of the same thickness to a production schedule for producing differential coatings normally requires a line stoppage to change the nozzle size of at least one of the jet finishing knives.
It previously has been proposed that multiple coating weights can be obtained using a pair of opposed jet knives for two side hot dip coating. U.S. Pat. No. 3,459,587 issued to D. L. Hunter, et al. discloses higher strip speeds, reduced gas pressures in the jet knives and a greater distance between the jet knives and the strip can produce heavier coating weights. Conversely, lower strip speed, higher gas pressure in the jet knives and a shorter distance between the strip and the jet knives can produce lighter coating weights. This patent further discloses that for given strip speed, gas pressure and distance between the strip and jet knives, coating weight can be varied using different orifice heights in the nozzles of the jet knives. Increased orifice height decreases coating weight due to the increase of gas passing through the larger orifice opening.
Varying one or more of the above parameters to produce different coating weights has not been very successful. Line speeds generally cannot be varied since they are limited by the heating capability of the furnace in the coating line. It is difficult to maintain good coating quality if the distance between the strip and nozzle is not maintained at the preferred distance determined for a given coating line. It is difficult to be able to vary and then accurately maintain constant gas pressure passing through the jet knives to produce different coating weights. Temperature changes to the gas, thermal expansion of the nozzle orifice, coating metal splatter into the orifice, etc. may cause the gas pressure to fluctuate from time to time. Finally, it is not practical to produce different coating weights by changing gas pressure while maintaining a constant orifice height. Producing light coatings using a large orifice opening may be limited by insufficient supply of the jet finishing gas. Using a small orifice opening for producing heavier coatings may result in poor surface appearance i.e. "jet lines".
It previously has been proposed to use multiple jet knives mounted for rotation so that either one of the knives could be used for controlling the weight of the liquid coating. The knives are identical, the extra one serving as a replacement if the main knife becomes damaged or plugged from coating splatter.
Unlike the prior art, my invention utilizes knives having different sized orifices or nozzle openings so that by rotating a different knife into position, a different coating thickness can be placed onto the web. This arrangement solves the production scheduling and inventory problems referred to above. It permits a production schedule to include a variety of coating weights or differential coatings without any need for shutting down the coating lines to apply a different coating weight to the web. Furthermore, each coating weight can be accurately maintained because those parameters affecting coating weight do not have to be changed. To change coating weight, the operator observes the tail end of a web receiving a first coating weight as it passes through the coating pot. He then rotates the jet knives until a nozzle having the appropriate orifice height is adjacent the passing web for the next lot of material requiring a second coating weight.